The invention relates to a steam box for a paper machine, the steam box comprising a steam chamber, a main steam zone, and at least one side steam zone, wherein steam is supplied to the main steam zone from the steam chamber, and also including at least one adjustable side steam zone valve through which steam is supplied to the side steam zone.
A paper machine utilizes steam boxes for blowing steam against a paper web. Among other things, the steam boxes enable the capacity of the paper machine to be increased. A steam box may be arranged, for example, in a press section and/or a wire section of the paper machine, whereby the steam box may be used, for example, for increasing the temperature of the paper web, and thereby press nips and dewatering equipment can remove warmed water more easily; consequently, the dry solids content can be increased and the drying capacity of the drying section improved. Furthermore, if desired, the steam box may even be used for correcting the moisture profile of the paper web. The steam boxes may also be used in many other parts of the paper machine, such as the drying section, calender section and paper converting machines.
U.S. Pat. No. 4,662,398 discloses a steam box of a paper machine wherein steam is supplied through a steam supply pipe to a steam chamber inside the steam box. In a cross direction of the web, the steam box is divided into adjacent chambers, and steam is supplied to each chamber from the steam chamber through chamber-specific and adjustable valves. Each chamber comprises a diffusion plate through which the steam is allowed to affect the paper web. The valves enable the amount of steam flowing into each chamber to be controlled chamber-specifically, thus enabling the cross direction profile of the paper web also to be adjusted.
FI publication 963 583 discloses a steam box of a paper machine wherein the valves controlling the amount of steam are arranged outside the steam box, and steam is supplied to each chamber through a separate, chamber-specific pipe. The pipes, however, cause rather great flow and temperature losses, so it is extremely difficult and unreliable to adjust the cross direction profile of the paper web by using the steam box at issue.
In the above-disclosed solutions, cold air is carried along the paper web, for example, between the paper web and the steam box, which decreases the effect of the steam on the paper web and makes the cross direction profile of the paper web more difficult to adjust. This problem has been solved in a known steam box such that in addition to the actual main steam zone, the steam box comprises at least one side steam zone. The tasks of the steam box, i.e. increasing the capacity of the paper machine and/or correcting the moisture profile of the paper web, for example, are implemented mainly by using the main steam zone. The side steam zone is located within the steam box prior to the main steam zone in the direction of travel of the web. By utilizing steam, a curtain is formed by the side steam zone to prevent air from being carried along with the web between the steam box and the paper web. The side steam zone may also be used for preheating the paper web and for correcting a potentially distorted moisture profile of the paper web. The side steam zone may also be located after the main steam zone in the direction of travel of the web, in which case the purpose of the side steam zone is to prevent the steam supplied to the main steam zone from escaping from between the web and the steam box.
U.S. Pat. Nos. 4,915,788 and 5,059,285 disclose a steam box to be arranged in the wire section of a paper machine. As seen in the direction of travel of the web, the steam box comprises three steam zones from which steam is supplied against the web. The front and rear sides of the steam box comprise side steam zones and a main steam zone therebetween. The purpose of the side steam zones is to prevent steam from exiting between the steam box and the web. Steam is supplied to the side steam zones through a chamber, to which chamber, in turn, steam is supplied through apertures in the wall of a steam chamber of the steam box. The solution cannot be used for adjusting the amount of steam supplied by the side steam zone, which would be necessary when, for example, the grade of the manufactured paper or the speed of the paper machine changes.
It is thus known to supply steam to the chamber supplying steam in the side steam zone by conveying the steam from the steam supply pipe of the steam box through a channel outside the steam box. An adjustable valve is arranged in the channel, either outside or inside the steam box. The valve is manually adjustable because the channels outside the steam box take space and no valves of different kind would fit in the space reserved for the steam box in a paper machine. Furthermore, the extremely difficult conditions surrounding the paper machine also affect the valve outside the steam box; hence, strict requirements are thus set for the structure of the valves, which also need quite extensive maintenance. The valve placed inside the steam box is particularly difficult to adjust since it does require the steam box to be shut down, cooled and opened in order to enable adjustments to be carried out in the first place.
A steam box of the invention is characterized in that steam is arranged to be supplied to the side steam zone through the side steam zone valve from the steam chamber, and that the side steam zone valve is arranged inside the steam box.
In accordance with the invention, a steam box comprises a main steam zone and at least one side steam zone through which steam is blown to a paper web, and the side steam zone is located in a side section of the steam box and is operable to form a curtain to prevent air from entering between the steam box and the paper web from outside the steam box and/or to prevent steam from exiting between the steam box and the paper web. Steam is supplied to the side steam zone from a steam chamber of the steam box through an adjustable valve, the valve being arranged inside the steam box. In a first preferred embodiment of the invention, the valve for adjusting the amount of steam supplied to the side steam zone is operable to be adjusted by a remotely controlled actuator. In accordance with a second preferred embodiment, the actuator for the valve adjusting the amount of steam supplied to the side steam zone is arranged in the same space inside the steam box as actuators adjusting the valves of the main steam zone. In a third preferred embodiment, the valve adjusting the amount of steam supplied to the side steam zone is arranged in the steam chamber of the steam box.
An advantage of the invention is that the solution in its entirety takes reasonably little space. The steam box has a simple structure and, arranged inside the steam box, the valves are protected against the difficult conditions surrounding the paper machine. Furthermore, a remotely controlled actuator may be arranged in connection with the valve adjusting the side steam zone, which means that in order to adjust said valve, it is unnecessary to stop the entire paper machine, such that the adjustment can be automatically carried out quite easily and cost-efficiently.
In connection with this description, in addition to paper, the term xe2x80x9cpaperxe2x80x9d also refers to paper board, tissue, and pulp.